Heretofore in drilling wells whose wellbores passed through a permafrost zone, water-based drilling fluids have often been employed. These drilling fluids contain a major amount of water and therefore are a freezable liquid.
It is undesirable to leave freezable liquids in the permafrost zone of a wellbore because, over the life of the well, the freezable liquid might become frozen.
Accordingly, it has been proposed to leave primarily only essentially nonfreezable liquids in the wellbore in the area of the permafrost zone. Procedures have been devised for displacing the freezable liquid from the wellbore and replacing same with an essentially nonfreezable liquid. In this invention, the term "essentially nonfreezable liquid" means that the liquid can contain some water, but generally will contain not more than about 30 volume percent water so that even should all this water, when properly distributed, freeze, there will be substantially no adverse affects to the well itself.
Initially, it was thought that a roughly one to one displacement process could be carried out, i.e., one volume of nonfreezable displacing roughly one volume of freezable liquid.
However, it has been found in actual practice that because of density differences between the freezable and nonfreezable liquids combined with the effect of gravity on the liquids as they pass downwardly in the wellbore, because of the difference in interfacial tensions between the freezable and nonfreezable liquids, and due to varying and often unknown rheological factors (for example, laminar flow, turbulent flow, and various gradations between these two types of flow in different parts of the wellbore at the same time) substantial amounts of the freezable liquid can be bypassed by the displacing nonfreezable liquid thereby failing to provide the desired result of essentially complete displacement of the freezable liquid from the permafrost zone of the wellbore.
Because the nonfreezable liquid is normally hydrocarbon based, it is highly desirable from an economic point of view as well as from an efficiency of operation point of view, to obtain essentially complete displacement of the freezable liquid using a minimum amount of nonfreezable liquid. Thus, the use of large excesses of nonfreezable displacement liquid in relation to the freezable liquid, i.e., using the nonfreezable liquid as a wash liquid, is not a viable alternative.
Further, in actual practice it has been found that in monitoring returns from a well to determine when essentially all of the freezable liquid has been recovered at the earth's surface, it is important from an accuracy point of view to calibrate the monitoring equipment on not only the water content but also the solids content of the liquid being monitored.